Problem: The scale on a map is 7cm : 10km. If the distance between two cities is 40km, how far apart in cm are the two cities on the map?
Solution: The scale means that every 7cm on the map corresponds to an actual distance of 10km. An actual distance of 40km is the same as 4 $\cdot$ 10km. The distance between the two cities on the map is 4 $\cdot$ 7cm, or 28cm.